I. Field of the Invention
The present invention relates to coating compositions adapted for application to film substrate materials such as therm plastic films, for example, polyolefin films such as polyethylene, polypropylene, polybutene, copolymers thereof and the like; polyester films such as, for example, polyethylene terephthalate, whereby certain properties of such coated films are improved. Specifically, uncoated polyolefin films such as oriented polypropylene for example, have an extremely narrow heat seal range, whereby such uncoated films exhibit a tendency to disorient and shrink when attempts are made to heat seal such films. The coating compositions of the present invention, when applied to such a substrate film, improve the heat seal properties thereof by broadening the heat seal range and lowering the minimum heat seal temperature requisiste to achieve satisfactory seals. Additionally, the coating compositions of the present invention improve the resistance of films coated therewith to the transmission of gases and vapors. The coated films exhibit excellent optical properties and may be readily heat sealed to other films such as polymer coated cellophane and the like.
II. Description of the Prior Art
The prior art discloses that multipolymers characterized by having a high vinylidene chloride content can be used advantageously as surface coatings for thermoplastic films such as, for example, oriented polypropylene films. As hereinbefore noted, these coatings contribute improved heat sealability, excellent optical properties, improved resistance to the transmission of gases and vapors, and heat sealability to coated cellophane for example.
In the past, such coating compositions have been applied to film substrates utilizing two basic coating application techniques: (1) coating from a solution of a high vinylidene chloride content multipolymer and a suitable organic solvent and (2) coating utilizing a dispersion or latex of discrete polymer particles of roughly 0.01 to 1 micron in diameter in water, such particles being stabilized by surface active agents.
The major advantage of the organic solvent coating technique, i.e. solution coating, is the case of film coating formation. Since the polymers are mono-molecularly dispersed in a suitable solvent, coherent films, i.e. coatings with relative freedom from voids ("pinholes") are formed by intermingling of the molecules as the solvent evaporates on drying. However, the disadvantages of such solution coating techniques are many and varied. Suitable organic solvent systems for high vinylidene chloride content multipolymers are relatively expensive and quite flammable. Solvent recovery systems are necessarily a requisite for economical operation with such coating systems. Further, unless very low molecular weight polymers are used, only dilute, i.e. on the order of about 20% solids, solutions can be employed because of the processing problems which would otherwise be encountered with the high molecular weight - high viscosity polymer solutions. Furthermore, high vinylidene chloride content multipolymers tend to retain solvents tenaciously, thus requiring extensive drying times for anything except extremely thin coating layers. U.S. Pat. No. 3,397,079 contains a disclosure relating to the employment of an organic solvent coating technique for this type of multipolymer system.
The latter type of coating system referred to hereinabove i.e. an aqueous dispersion or latex coating system, is disclosed in U.S. Pat. No. 3,459,582. A latex, by definition, is a dispersion of spherical, polymeric particles in the range of from about 0.01 up to about 1.0 micron (0.004 - 0.04 mils) diameter in water. Latex coating systems employed for coating application to film substrates generally consist of from about 50% to about 60% solids with about 2% to 3% of the solids being wetting and suspending agents. On drying the applied coating, the water evaporates and the particles fuse together at their contacting surfaces. By the very nature of such a latex coating process, only relatively thick coatings can be obtained. Conversely, when attempts are made to apply relatively thin coatings from a latex media, i.e. on the order of about 0.05 mils or less, such coatings are usually characterized by minute voids or pinholes therein and, additionally, commercially prohibitive, long baking must be employed. Additionally, it is not usually practical, due to the difficulties of obtaining uniform dispersion, to add any substantial amount of modifier ingredients to the latex coating system such as, for example, hot slip additives and other polymers.